Apparatus suited for extreme ultraviolet lithography, comprising a radiation source and a processing organ for processing the radiation from the radiation source, and a filter for suppressing undesired atomic and microscopic particles which are radiated by a radiation source.
The invention relates to an apparatus comprising a radiation source and a processing organ for processing the radiation from the radiation source, wherein a filter is placed between the radiation source and the processing organ, which filter comprises a plurality of foils or plates.
Such an apparatus may be used in the production of integrated circuits, that is to say in a lithographical application. The invention may also be applied in various other fields. For a good understanding of the invention, however, a lithographical application will serve well as illustration. Continuous attempts are made to make integrated circuits smaller and smaller in order to improve the processing speed of the integrated circuits.
According to the prior art, such integrated circuits are manufactured chiefly by using lithography with visible and ultraviolet light. With these known technologies, it is possible to manufacture integrated circuits that may be as short as 120 nanometers. The ultraviolet light used with said circuits has a wavelength of 193 nanometers. The known techniques do not allow a further decrease of the dimensions of the integrated circuits, and a possible solution is the use of lithography on the basis of extreme ultraviolet light. Such light has a wavelength of 13 nanometers. The known optical elements cannot be used at this wavelength. The known mirrors and lenses absorb too large a portion of the extreme ultraviolet light. In order to allow for this, the processing organ for processing the radiation from the radiation source is a multi-layer mirror which consists of 40 or more molybdenum layers alternating with silicon layers.
In such an apparatus for extreme ultraviolet lithography a laser plasma source is used to generate a plasma by heating an object by means of a laser source of high energy density, for example of at least 1011W/cm2. The object heated by the laser will function as source of secondary emission of mainly shortwave radiation. However, this will also release undesirable particles and atoms producing the effect of debris in the apparatus. The objective of the invention is to prevent the production of said debris.
WO 96/10324 discloses such an apparatus for the generation of radiation. This apparatus uses a fast rotating target which is heated by the laser source and which produces the secondary emission. Due to the kinetic energy of the particles formed from the plasma on the rotating target, this apparatus has a filtering effect in respect of the so-called macro-particles. However, trapping atoms, and in particular the fastest micro-particles, is not possible in this known apparatus.
U.S. Pat. No. 4,837,794 concerns a filter apparatus comprising a radiation source and a processing organ for processing the radiation from the radiation source, wherein a filter is placed between the radiation source and the processing organ, which filter comprises a plurality of foils or plates, including a baffle for diffusing hot gases and directing them away from a window of sight. Due to the placement of the baffle, the said window of sight is rather narrow.
It is the object of the invention to circumvent the drawbacks of the prior art. According to the invention, this is realized by the apparatus, for example for extreme ultraviolet lithography, comprising a radiation source and a processing organ for processing the radiation from the radiation source, wherein a filter is placed between the radiation source and the processing organ, which filter comprises a plurality of foils or plates, which is characterized in that each foil or plate essentially points in the radial direction when viewed from the radiation source. Surprisingly, it has been shown that this very simple measure not only makes it possible to trap atoms and micro-particles, but also clusters of such micro-particles, respectively the smallest macro-particles.
Quite advantageous is that the apparatus according to the invention shows no limitation with respect to the effectively usable angle of sight due to the fact that the apparatus embodied with the filter according to the invention provides full optical transparency.
A first preferred embodiment of the apparatus according to the invention is characterized in that the foils or plates are positioned in a honeycomb construction.
A second preferred embodiment of the apparatus according to the invention is characterized in that the foils or plates are cone-shaped and are positioned concentrically.
Preferably, in the radial direction the foils or plates are positioned such as to be evenly distributed in relation to one another.
Such an apparatus is used with a buffer gas in which the radiation source and the processing organ are placed. Appropriately, the distance between the radiation source and the filter""s proximal end in relation to the radiation source is then selected subject to the pressure and the type of buffer gas. A very suitable choice of buffer gas is krypton, whose pressure is 0.5 Torr, and the distance between the radiation source and the proximal end of the filter is 5 cm. This setting affords sufficient opportunity for the particles to be trapped in the filter to take on the temperature of the buffer gas, for example room temperature, thereby sufficiently reducing the particle""s velocity before it enters the filter.
It is further desirable to select the length of the filter, which is formed by the distance between the filter""s proximal end and its distal end in relation to the radiation source, subject to the pressure of the buffer gas and the form of the filter. Especially the gas pressure determines the mean free path length for the particles to be trapped; a lower gas pressure corresponds to an increased free path length. This can be partially compensated by the form of the filter. For example, using the above-mentioned honeycomb construction provides a larger surface area, affording greater opportunity for the particles to actually be trapped.
It has been shown that good results can be obtained when the length of the filter is at least 1 cm. This filter length corresponds with a usual gas pressure of, for example, 100 mTorr.
As already mentioned above, the apparatus is operational at room temperature. The measure of maintaining the filter at a temperature which is approximately below room temperature allows the residence time of the atoms and particles trapped on a foil or plate to be increased, and accordingly the effectiveness of the filter to be improved.
It is further desirable that the number of plates in the filter should be adjusted subject to the thickness of each plate and the desired optical transparency of the filter as determined by the formula       d          d      +              d        f              xc3x97  100  ⁢      xe2x80x83    ⁢  %
in which d=the distance between two plates of the filter at the side of the radiation source; and df=the thickness of a plate of the filter.
In this way the light output of the integral apparatus can be maintained at an adequate level, while the effectiveness of the filter can still be 100%. The apparatus is then preferably characterized in that the number of plates is adjusted such that the distance between two plates is approximately 1 mm.
The effectiveness of the filter may be improved further by roughening the surface of the plates.
The invention is further embodied in a separate filter for suppressing undesirable atomic and microscopic particles emitted by a radiation source, wherein a plurality of plates are positioned substantially parallel in relation to one another, for trapping atomic and microscopic particles on their respective surfaces.
Such a separate filter is characterized in that the plates are directed radially from the radiation source.